Support is requested for a gas chromatography-mass spectrometry system (Hewlett-Packard 5988A) that will complement existing instruments in the Mass Spectrometry Center at the Children's Hospital of Philadelphia. The new system is needed to accommodate a rapidly growing demand for GC-MS as an essential component of numerous PHS grants (see above list) currently held by investigators at the Children's Hospital. Each of these funded proposals requires mass spectrometry for the measurement of stable isotopic abundance in a metabolite of interest. This measurement is necessary either: (a) to measure flux through a metabolic pathway, particularly through pathways of nitrogen metabolism, when 15N is used as a tracer; (b) to use stable isotopes to measure the in vivo turnover of amino acids and bile acids in the human neonate; and (c) to quantitate very low levels of metabolites with a stable isotopic dilution assay. Mass spectrometry is the only practical technique with which enrichment can be measured in such species as 15N or 13C, especially when the sample size is very small. The existing two gas chromatography-mass spectrometry systems were purchased by the Stokes Institute of the Children's Hospital, NIH funds having supported only supplies and personnel. Unfortunately, these two GC-MS systems are in almost constant use because of the heavy demands imposed by funded proposals. Such heavy utilization inevitably strains instrumental performance and favors the ultimate obsolescence of the equipment. The acquisition of the proposed system will allow the successful consummation of the funded research and also will abet the development of future projects.